In the manufacture of semiconductor devices, it is becoming increasingly important to detect electrical defects, such as open or short circuit defects, as early as possible in the fabrication process to shorten the development cycle and improve product yield. Advanced wafer inspection and review systems based on a Scanning Electron Microscope (SEM) system have been commonly used for detecting and reviewing electrical defects as a voltage contrast image.
FIG. 1 is a diagrammatic representation of an electron beam apparatus 100 (SEM) in accordance with one embodiment of the present invention. The SEM system 100 includes an electron gun (101 through 103) that generates primary electron beam 104. The SEM system 100 also includes one or more condenser lenses 105 and an objective lens 109 that focuses and directs the primary electron beam 104 substantially toward a specimen 113. The SEM system 100 also includes at least a set of deflectors 110 to raster scan the primary electron beam over an area of interest on the specimen 113. The SEM system 100 also includes an in-lens detector 107 arranged below a final aperture 106 to detect signal electrons 111 (including secondary electrons SE and/or backscattered electrons BSE) emanating from the specimen surface 113. The SEM also includes an image generator (not shown) to generate an image from the emanated electrons that can be displayed and/or stored in a computer system.
The fraction of total signal electrons (SE+BSE) produced by the primary electron beam, defined as total electron yield, depends on the material and the primary electron beam landing energy (LE). FIG. 2 is a chart which shows the curve of the total electron yield (201) as a function of primary beam LE. For most materials, there will be two LE points E1 (203) and E2 (204) where the curve (201) crossover the yield of 1 (202). E1 and E2 are called the first and second crossover point respectively. E1<LE<E2 defines a positive mode region; LE<E1 and LE>E2 defines a negative mode region. This enables SEM to reveal features of different electrical properties in voltage contrast image.
If features on a wafer are supposed to be well grounded or virtually grounded (by being connected to a very large capacitor), the charges induced during the process of primary electron scanning will be released easily or at least kept at a very low level. If a feature becomes defectively open or floating, charges will be accumulated. The open/floating feature will appears relatively dark if the primary electron landing energy is in the positive region (E1<LE<E2), or relatively bright if in negative region (LE>E2).
If the normal features on wafer are supposed to be open or floating, such as a gate, the charging induced by primary electron scanning accumulates. If a feature shorts to ground or virtual ground abnormally, no charge or less charge can be accumulated on the feature. This will make the feature appear bright in relation to other features on the wafer if primary electron landing energy is in the positive region (E1<LE<E2) or the feature will be dark in relation to other features if in the negative mode region (LE>E2).
Generally defective contacts to NMOS and PMOS can be identified from a voltage contrast image by using the SEM. Conventional approaches focus on ways to manipulate the associated pn-junctions into different kinds of modes or statuses. For instance, if scanning NMOS contacts in positive mode and PMOS contacts in negative mode, the corresponding pn-junctions will be set to forward biased mode. In both cases, charges on the normal contacts will be kept to a relative low level as excessive charges will be released to the substrate via the pn-junction. An open contact accumulates charge, thus it is distinguishable from the normal contacts in the SE signal or voltage contrast image.
For a deep trench DRAM, due to the contact materials used and device layout, conventional approaches exhibit ineffectiveness or insensitivity in identifying open bit contacts at the bit contact layer. Accordingly, what is needed is a system and method for overcoming the above-identified issues. The present invention addresses such a need.